Did you see the link I posted Evan? It gives some answers to your question, other than that it's a matter of trial and error. There's always exceptions to the rule but eventually a hard winter like the last one we had will kick those exceptions into plant heaven. We get an exceptionally cold winter every 20 years give or take a little, at times down to -20C. Some winters can be mild with often 8C in the day and night, one winter we had only 4 frosts down to -5C.
For example: Plants which are said to be hardy to -5 or -6C but have endured worse ..
I had a Pittosporum 'Eilsa Keighley' in the ground for about 10 years, it was about 10 feet tall and very happy. Dead.
I had Pittosporum Tom Thumb in the ground for the same time, it was a nice mature shrub. Dead.
I had Acacia dealbata in the ground for the same time. It was huge maybe 35 feet tall, perhaps too big as it was smothering other plants. Almost dead! The trunk was very thick but the tree was frosted so much the leaves died, and the trunk did too as it turned from a bue-grey colour to a funny brown-red colour. But, the roots travel far and there's masses of them, now I have plants growing off the roots all over the place! I also had seeds germinating and growing, it has made plenty of seed before but they never grew on the ground. Nature was making sure that tree lived but I now have a problem!
I have several Phormiums, one died but it had ground cover around it so was maybe kept too wet for very hard frosts. Pink Stripe was a large plant, slapping my front window in high winds, it had a huge base but the plant looked dead. It started to regrow from the base but I decided this was my opportunity to dig it out. Others I have in a very sandy place, almost sandstone in places, they had the leaves killed too and regrew from the base so all was not lost. Pampas grasses I have were killed back, chopped the plants down to the base and they have regrown, same with Cordyline australis I grew from seed and were looking great, I lost one Cordyline and two Pampas grasses. If we had another very hard winter this time I doubt they would live as they were weakened and haven't had time to grow strong again.
My Trachycarpus fortuneii which I grew from a seedling survived! Most of the leaves were half frosted, and many of those which grew this year were scorched but it's quite a big palm now and has been in the ground since 2004 when it was about 5 years old. It's really the only palm which will stand a chance here.
